2018-144 Update on Sewage Release at Pecan CreekDate: October 5, 2018 Report No. 2018-144
INFORMAL STAFF REPORT
TO MAYOR AND CITY COUNCIL
SUBJECT:
Provide an update on the recent sewage release to Pecan Creek.
EXECUTIVE SUMMARY:
The City of Denton’s wastewater collection system experienced a failure in a 21 inch line that
conveyed waste along Pecan Creek to the Pecan Creek Water Reclamation facility, resulting in the
release of approximately 2.6 million gallons of domestic sewage. The purpose of this report is to
provide the council with additional background and status of this issue.
BACKGROUND:
Due to recent heavy rainfalls, a section of the creek bank along Pecan Creek suffered a severe
slope failure. An area approximately 70 feet long, 20 feet wide and 20 feet high collapsed, carrying
a 21-inch diameter sewer interceptor with it. The failure occurred approximately 1,800 feet
southeast of the Pecan Creek Water Reclamation Plant headworks. Watershed protection staff
located the failure at approximately 1:30PM Monday October 1. Utilities staff were on site by
2:00PM and immediately began work to stabilize the slope and stop the overflow. Water
distribution and wastewater collection staff were able to place a plug in the line at a manhole
upstream of the failure, successfully bypassing the flow to a parallel 24-inch diameter line at
11:30PM Monday evening. Staff performed non-stop earthwork activities to prevent any further
slope failure in the area, completing temporary stabilization at approximately 2:00 p.m. Tuesday.
Wastewater collection staff are currently on site 24-hours a day to monitor the status/condition of
the plug, to prevent it from becoming dislodged from the pipe, and to observe any further erosion
by water flow in the creek.
Also on Monday, Utilities Administration enlisted the services of a 3rd party structural engineer to
assist with design and recommendations for both temporary and permanent stream bank
stabilization. A subsurface geotechnical investigation, led by a geotechnical engineer, commenced
Tuesday afternoon. The purpose of this investigation is to determine soil properties and the depth
to bedrock, data which are necessary to prepare an engineered design of permanent repairs.
Analysis and conceptual design are currently ongoing, pending the receipt of the geotechnical
report.
The current conceptual plan for repair includes several complementary solutions. As the soil that
once contained the pipe was completely dislodged, we currently intend to install drilled concrete
piers to support the replacement pipe, to allow streambank restoration construction to occur
immediately adjacent to the pipe. Repair of the stream bank will require some form of structural
wall or slope, which will be resistant to future erosion. The most effective type of wall or slope
will be determined based upon the forthcoming geotechnical report.
The team of professionals responding to this failure included City staff members of Water,
Wastewater and Drainage divisions, some working over 30 continuous hours. Additional
Date: October 5, 2018 Report No. 2018-144
equipment coordination was provided by several Solid Waste staff members until late into the
night Monday. The Wastewater Utility sincerely appreciates the collective effort of all staff
involved, as well as the supply of necessary construction equipment from several departments
without hesitation. Some photographs are included below to show the extent of this challenging
situation.
Staff continue to monitor impacts to the stream, and monitoring data indicates that the stream is
recovering. Aeration and monitoring will continue until the stream returns to normal conditions.
Initial failure conditions.
Date: October 5, 2018 Report No. 2018-144
Slope stabilization activities
Geotechnical investigation of temporary stabilization
Date: October 5, 2018 Report No. 2018-144
Completed temporary slope stabilization
STAFF CONTACT:
Frank Pugsley
Deputy Director of Water and Wastewater Utilities
940-349-8086